In a turbomachine of the kind specified above, a compressor stator is fitted with at least one stator vane stage that is of variable pitch, i.e. the position of the vanes in the flow section is adjustable. More precisely, the angle of attack of the vanes is controlled as a function of operating conditions by a servo-control system which causes a rotary actuator ring outside the casing to move, the actuator ring being connected to said vanes by respective cranks.
Conventionally, such an actuator ring includes radially-adjustable centering pads whereby it is positioned around the casing by pressing thereagainst. Each centering pad presses against a track on the casing, i.e. on a projection defined on its surface, the projections being machined so that their contact surfaces occupy a cylindrical surface of axis that coincides with the axis of the turbomachine.
The adjustments for ensuring a satisfactory coaxial position of the actuator ring are difficult to perform and take time. In addition, once the adjustment has been performed, it is necessary to give clearance to the sliding assembly between said pads and said projections in order to accommodate the variation in dimensions due to temperature variations in operation. This necessary clearance of about 0.7 millimeters (mm) to 0.8 mm goes against the desired centering.
In another design, U.S. Pat. No. 4,130,375 describes an actuator ring that moves circularly only, the ring being made up of two superposed rings. The inner ring constitutes a track ring mounted stationary on the casing by means of radially-extending studs, and the outer ring is the actuator ring proper, being attached via links to vane-actuation cranks and running on the track ring via a system of wheels. Such a system is heavy and bulky, other things being equal.